The present invention relates to radio frequency (RF) communication systems, and more specifically, to apparatus and methods for rejecting receive band signals in RF mixed-signal amplifiers.
Wireless communication, such as cell phones for voice and data, has become extremely popular. Currently, several wireless schemes are in use, including GSM, TDMA, and CDMA. Of these, CDMA appears to be emerging as the standard in the U.S., European and Asian markets. CDMA often requires RF transmissions using both phase and amplitude modulation. The efficiency and power consumption of the power linear amplifiers used to generate an RF signal in either a CDMA cell phone or base station are therefore extremely important. Use of low efficiency linear amplifiers is detrimental for several reasons.
Such amplifiers tend to burn a significant amount of energy which is problematic, particularly in a battery operated cell phone. Power consumption is also problematic in base stations. The heat caused by many low efficiency amplifiers in a base station can cause components to fail, thus reducing reliability. The linearity of the power amplifier is also important. In a base stations where the transmission of multiple signals occurs simultaneously, amplifiers characterized by poor linearity may cause the inadvertent mixing of these signals.
A number of types of amplifier classes can be used in RF communication systems, including Class A, Class AB, Class C, Class E, Class F, and Class D (sometimes referred to as digital amplifiers). Each of these types of amplifiers, however, have significant problems when operating in the RF range. For example, Class A and Class AB amplifiers have very poor efficiency. Classes C, E, F and D amplifiers have improved power efficiency when compared to Class A and Class AB type amplifiers, but they are not suitable for linear applications. Further, Class E amplifiers suffer from severe overshoot problems at their output hence limiting their usefulness. Class F amplifiers exhibit relatively good output switching characteristics with a repeating input signal. But with a non-repeating input signal, such as those normally encountered in a cellular phone or base station, the problems caused by harmonics become overwhelming. As a result, Class F amplifiers are difficult to use in RF linear power amplifier applications.
Conventional class D amplifiers have linear operating characteristics and are generally highly efficient at lower frequencies but have heretofore been subject to several drawbacks at higher frequencies. Most notably, at higher frequencies such as RF they exhibit switching problems at their output transistors. As these transistors switch on and off rapidly, switching transients including high levels of current and voltage are developed at the output, causing overshoot and undershoot.
In view of the foregoing, it is desirable to provide an efficient digital amplifier capable of operating in the RF range.
According to the present invention a band pass amplifier is provided having a frequency selective network in a feedback loop. An analog-to-digital converter is coupled to the frequency selective network. A switching stage is coupled to the analog-to-digital converter for producing a continuous-time output signal. The switching stage includes at least one resonance circuit configured to resonate at a resonance frequency and thereby generate at least a portion of the continuous-time output signal. A continuous-time feedback path continuously senses and feeds back the continuous-time output signal to the frequency selective network.
A switching stage for a band pass amplifier is also provided. The switching stage includes first and second switches in a half-bridge configuration, each of the first and second switches having parasitic capacitances associated therewith. The switching stage further comprises a plurality of inductors. Two resonance circuits are formed from the parasitic capacitances and the inductors, the two resonance circuits being configured to alternately resonate at a resonance frequency.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.